1. Field of the Invention
The invention is related to an innovative process for membrane electrode assembly, from formulating with raw materials, and preparing electrodes of planar SOFC-MEA containing anode and electrolyte substrate slurry, to making green tape of electrode by tape casting process. The green tape is cast into green substrate by lamination process, and then finished into high integrity green substrate by vacuum hot press system (VHPS). Through calcinations and sintering, the green substrate can be made into electrode substrate with high mechanical strength, controllable micro-structural characteristics (porosity/gas permeability), thickness, and dimensions. Further, screen-printing, sputtering coating, spin coating, and spraying processes are alternatively employed to enable the production of high-performance SOFC unit cell. Their application to SOFC assures high reliability, durability and low degradation rate of the unit cell. The raw materials the invention refers to are YSZ/GDC/YDC/LSGM electrolytes, NiO+YSZ/GDC+NiO/YDC+NiO/LSGM+NiO anode materials and LSM/LSCF cathode materials. But they are not limited to the above materials.
2. Description of the Prior Art
SOFC has high conversion efficiency, low noise, low pollution, high reliability and fuel diversity, as well as the potential to replace internal combustion engine in solving energy shortage issue. Especially when fossil fuels are in short supply, and hydrogen, natural gas and LPG become the alternatives; SOFC is a very important energy conversion device and plays a key role in the new energy era.
The critical goals for the Planar Type Solid Oxide Fuel Cell—Membrane Electrode Assembly (SOFC-MEA) currently under development include high performance, high durability, high stability and low degradation rate of MEA. To achieve the above goals, the key factors are the MEA materials and structure design. Changing the materials and MEA structure also changes MEA properties. For materials, the electrolyte is mainly 8YSZ, which operation temperature depends on the supported substrate structure. Electrolyte Supported Cell (ESC) is operated at temperature range of 800˜1000° C. with electrolyte thickness of 150˜300 μm and termed as first generation SOFC. The second generation SOFC adopts Anode Supported Cell (ASC), with operation temperature range of 650˜850° C. and electrolyte thickness of 10 μm. NiO+8YSZ are the anode materials for ASC/ESC with thicknesses of 50˜60 μm (ESC) and 500˜1200 μm (ASC). The cathode materials are mainly LSM and LSCF having thickness 30˜60 μm. New electrolyte materials and cathode materials are actively under development in many international laboratories. It is expected the new materials lower SOFC-MEA operation temperature to 500˜700° C. Then, the modules and parts for SOFC Stack, like inter-connector, can use metal materials to replace ceramic materials. The advantages will include easy fabrication, increased mechanical strength/stability/durability and reduced cost. Increasing SOFC marketing competitiveness and penetration will create huge niche for SOFC industry. The technical development in universities and national laboratories emphasizes materials development in the hope to lower resistance, increase ionic conductivity/electric conductivity, and increase SOFC powder density. On Nature magazine, there are many publications on new electrolytes like LSGM, YDC, LSGMC and 10ScCeSZ, or new cathode materials like LSM/LSCF/LSF/LSC/LSCM/BSCF/SSC. In industry, the emphasis is on material processing technology and performance stability. If the right materials are selected to achieve the desired properties and high reliability, and work with SOFC-MEA process and structure design, MEA mechanical/chemical stability, durability, energy conversion and powder output will be upgraded. Then, SOFC will become the best device in energy conversion.
Many foreign laboratories have been through 15˜20 years investing in R&D and testing SOFC performance, also overcoming technical barriers and creating business opportunities. There is collaboration between corporation and national laboratory and joint venture is formed. Resource exchange, integration, merger or cooperation are taking place in Europe and America, such as ECN and InDEC, or H.C. Stark, IKTS and Karafol/Straxera/Webasto, NETL/SECA, EPFL and HT Ceramix. In the US, their national laboratories such as PNNL et al. and six large companies including Simens Westinghouse and GE and Delphi et al. are collaborating on verification of technical reliability, cost analysis and reduction to establish SOFC industry.
The SOFC-MEA related materials that have potential for commercialization include those mentioned previously, such as 8YSZ as electrolyte, NiO+8YSZ as anode materials, LSM/LSCF and LSF/LSC as cathode materials. As for MEA processes, they are rarely published and proprietary to the companies who develop and do not want to patent the technologies because they are afraid their interest be hurt when the patented technologies are copied or modified by others.
Current SOFC-MEA processes are based on tape casting to produce green tape of electrode first. The following lamination process allows adjustment for the thickness and geometry for green substrate. Then, the calcinations and sintering process produces electrode substrate or half cell substrate (including electrolyte layer and support electrode layer). At last, the screen-printing technique is used to build cathode layer onto half cell board, which completes the production of SOFC-MEA. The main drawbacks for the SOFC-MEA produced in this way are: insufficient mechanical strength, poor stability and durability (poor resistance to Redox Cycling/Thermal Cycling). Under the basic requirement for porosity (beneficial for gas-solid reaction mechanism) in cathode and anode, it is necessary to sacrifice mechanical strength. Later, this will cause rupture and failure to the assembled cell stack. Such a drawback hinders the development of perfect structure for SOFC and prompts immediate slurry.